1. Field of the Invention
This invention relates to a turbine mechanical output computation device for computing the mechanical output of a turbine which rotationally drives a generator, and a gas turbine control device equipped with the turbine mechanical output computation device.
2. Description of the Related Art
In controlling a lowNOx combustor in a gas turbine, it is necessary to maintain the ratio between the amounts of fuel and combustion air, which are supplied to the combustor, i.e., fuel-air ratio, at a value in a predetermined range. For this purpose, the amount of fuel needs to be detected accurately and promptly.
In the fuel-air ratio control of the gas turbine by a gas turbine control device, the mechanical output of the gas turbine has been taken as an indicator of the amount of fuel. To measure the gas turbine mechanical output, generator output (generator effective power) has been used. That is, based on the relationship of the equation shown below, generator output is used as a signal representing the amount of fuel. The reason for the use of generator output is that compared with a flow meter for measuring the amount of fuel, a power converter (generator effective power wattmeter) for measuring the output of a generator is characterized by high precision, high speed detection, and high reliability (multiplexing is also easy).Amount of fuel=Amount of energy fed into gas turbine=Gas turbine mechanical output=Generator output
Japanese Patent Publication No. 1995-111148 discloses an example of an acceleration control method for a two-shaft gas turbine using a gas turbine rotational speed signal.
When a generator effective power wattmeter is used for measuring gas turbine mechanical output, as described above, all of gas turbine mechanical output is converted into generator output, in an ordinary stable operating state where the gas turbine rotates at a constant rotational speed. Thus, the following relationship holds, and no problem is posed:Gas turbine mechanical output=Generator output (Generator effective power)
Generally, however, even when the amount of fuel supplied to the combustor is nearly constant, and the generator is operated with constant output, there may be a case where disturbance occurs in an electric power system to which the generator is connected (i.e., such as switching of a power transmission system). In this case, current fluctuations occur in the electric power system, causing changes in effective power, reactive power, and power factor. That is, in the event of a disturbance in the electric power system or the like, the electric power system falls into a state where it cannot receive electric power stably, with the result that the relationship of the above equation is transiently impaired. In such a state, not all of the gas turbine mechanical output is converted into the generator output, and the portion of the gas turbine mechanical output not converted into the generator output is converted into the rotational energy of the gas turbine. As a result, the rotational speed of the gas turbine changes. In such a transient period, therefore, fuel-air ratio control deviates from the optimal state, thus presenting problems, such as the occurrence of combustion vibrations in the combustor. The occurrence of combustion vibrations becomes the cause of a misfire in or damage to the combustor.